In optical communication systems, photodetectors are used to convert optical signals into electrical signals. The most commonly used photodetectors are positive-intrinsic-negative (PIN) photodiodes and avalanche photodiodes (APDs).
A typical PIN photodiode includes an absorption layer of intrinsic, i.e. not intentionally doped, semiconductor material between a region of extrinsic, i.e. doped, semiconductor material of a first conductivity type, i.e. n-type or p-type, and a region of extrinsic semiconductor material of a second conductivity type, i.e. p-type or n-type, an arrangement that produces an electric field in the absorption layer. In operation in photoconductive mode, a reverse voltage is applied to the PIN photodiode to enhance the electric field in the absorption layer. Light incident on the PIN photodiode is absorbed by the absorption layer to generate current carriers, i.e. electrons and holes, in an absorption process. The generated current carriers are separated by the electric field in the absorption layer and drift toward the regions of extrinsic semiconductor material: holes drift toward the region of p-type semiconductor material, and electrons drift toward the region of n-type semiconductor material. The resulting photocurrent is proportional to the optical power of the incident light.
A typical APD includes a multiplication layer of intrinsic or lightly doped extrinsic semiconductor material, in addition to an absorption layer of intrinsic semiconductor material, between a region of extrinsic semiconductor material of the first conductivity type and a region of extrinsic semiconductor material of the second conductivity type, an arrangement that produces electric fields in the multiplication layer and the absorption layer. In operation, a large reverse voltage is applied to the APD to enhance the electric fields in the absorption layer and the multiplication layer. As in the PIN photodiode, light incident on the APD is absorbed by the absorption layer to generate current carriers, in an absorption process. The generated current carriers are separated by the electric field in the absorption layer, such that either holes or electrons drift toward the multiplication layer. The electric field in the multiplication layer is large enough that the holes or electrons acquire sufficient kinetic energy to generate additional current carriers through impact ionization. The generated current carriers, in turn, generate additional current carriers through impact ionization. Thus, current carriers are multiplied in an avalanche multiplication process in the multiplication layer, leading to a multiplied photocurrent.
The absorption layer of PIN photodiodes, and the absorption and multiplication layers of APDs are active layers of primary importance to device operation, as the absorption and avalanche multiplication processes responsible for the photocurrent occur predominantly in active regions of these active layers. However, many conventional methods of fabricating PIN photodiodes and APDs include steps that may introduce defects into an active region of an active layer. For example, during fabrication of PIN photodiodes and APDs having a mesa configuration, a mesa may be etched through an active layer to define an active region. During fabrication of PIN photodiodes and APDs having a planar configuration, dopant diffusion into an active layer may be used to define an active region. Alternatively, during fabrication of PIN photodiodes and APDs having a planar configuration, ion implantation into an active layer may be used to define an active region.
In attempts to achieve PIN photodiodes and APDs with improved performance characteristics, fabrication methods have been developed that avoid modification of an active region of an active layer, for example, through etching, dopant diffusion, or ion implantation.
During fabrication of PIN photodiodes and APDs having a planar configuration, a grading or buffer layer may be formed on an active layer, and dopant diffusion into a layer above the grading or buffer layer may be used to define an active region, as disclosed in an article entitled “Simple Planar Structure for High-Performance AlInAs Avalanche photodiodes” by Yagyu, et al. (IEEE Photonics Technology Letters, 2006, Vol. 18, pp. 76-78), in U.S. Pat. No. 5,001,335 to Takaoka, et al., in U.S. Patent Application No. 2005/0156192 to Ko, et al., and in U.S. Patent Application No. 2004/0251483 to Ko, et al. However, dopant diffusion can induce redistribution of doping profiles in epitaxially grown layers, such as the grading or buffer layer and the active layer.
During fabrication of PIN photodiodes and APDs having a mesa configuration, a mesa may be etched above an active layer to define an active region, as disclosed in an article entitled “A New Planar InGaAs-InAlAs Avalanche Photodiode” by Levine, et al. (IEEE Photonics Technology Letters, 2006, Vol. 18, pp. 1898-1900), in U.S. Pat. No. 6,756,613 to Yuan, in U.S. Patent Application No. 2005/0156192 to Ko, et al., and in U.S. Patent Application No. 2004/0251483 to Ko, et al.
The present invention provides a highly reliable photodiode having a mesa configuration, as well as a simple method of fabricating such a photodiode. Advantageously, an active region is defined without modifying an active layer through etching, dopant diffusion, or ion implantation. During fabrication of the photodiode, a grading layer is epitaxially grown on a top surface of an absorption layer, and a blocking layer, for inhibiting current flow, is epitaxially grown on a top surface of the grading layer. The blocking layer is then etched to expose a window region of the top surface of the grading layer. Thus, the etched blocking layer defines an active region of the absorption layer. A window layer is epitaxially regrown on a top surface of the blocking layer and on the window region of the top surface of the grading layer, and is then etched to form a window mesa.